Since its recent discovery, graphene has attracted much attention due to its properties, such as high electronic mobility, extraordinary thermal conductivity, great strength, flexibility and transparency. These properties make graphene an ideal candidate in many applications, such as in electronics, in energy, in touch screen and display technology and in sensors. Most of these applications will require a large-scale production of graphene. A conventional way of manufacturing graphene is by rearranging the carbon atoms in a Chemical Vapor Deposition (CVD) process. In fact, CVD, in combination with metal catalysts, has become the most preferred choice for large area production of monolayer graphene. However, most of the applications require graphene transferred onto different substrates. For example, European patent application U.S. Pat. No. 9,023,220B2 discloses a method of manufacturing a graphene monolayer on insulating substrates from CVD graphene synthesis.
Graphene being a one atom thick material (one million times thinner than an A4 piece of paper) makes the handling of this material extremely challenging and difficult. In particular, graphene is expected to have excellent potential application in sensors, such as NEMs (nanoelectromechanical) or MEMs (microelectromechanical), among others. In these applications, graphene needs to be suspended on cavities or on holes. For example, United States patent application US2013/0196463A1 US2013/0018599-A1 discloses a graphene nanodevice comprising a suspended graphene layer. The graphene membrane is said to be formed on a separate substrate and transferred onto a planarized surface. According to this disclosure, a thin graphene film can be grown by CVD on copper foil. Then a thin film of polymethyl methacrylate (PMMA) is spun onto the graphene surface. Then the PMMA/graphene/copper stack is soaked in a copper etchant to remove copper. The PMMA/graphene film is then transferred to the target substrate by immersing the target substrate in water and placing the PMMA/graphene film on top. The PMMA can then be removed by using acetone or thermal treatment. The resulting graphene membrane will adhere to the planarized surface via Van der Waals interaction forces.
Graphene is also expected to have excellent potential application in combination with substrates having at least one water-soluble layer (such as MoO3 or PEDOT). In these applications, in which a graphene film needs to be deposited on such substrates, a wet transfer of graphene, such as the one described in US2013/0196463-A1, is discouraged because the residual water would damage the substrate, dissolving it. In addition, the final step of the transfer process, immersing the target substrate in water, would seriously damage it.
César J. Lockhart de la Rosa et al. describe in “Frame assisted H2O electrolysis induced H2 bubbling transfer of large area graphene grown by chemical vapor deposition on Cu” (Applied Physics Letters 102, 022101 (2013)) a technique for transferring a layer of graphene grown by CVD on copper, based on mechanical separation of the graphene/copper by H2 bubbles during H2O electrolysis. The process is as follows: First, graphene is grown by CVD on copper followed by deposition of a support PMMA thin film. Then a polyethylene terephthalate (PET) supporting frame is placed on the top of the PMMA/graphene/Cu-sandwich. The PET-frame/PMMA/graphene/Cu-bundle is submerged into an aqueous solution and subjected to electrolysis for separating the Cu foil from the graphene by the H2 bubbling. The PET-frame/PMMA/graphene-bundle is then picked up and rinsed in several deionized water baths. Next it is placed on the SiO2/Si target substrate and left at room temperature until it gets dry. The PET frame is then removed by cutting. The PMMA is then dissolved by acetone. Gluing PMMA to PET frame is a complicated task because there is no adhesive element included in the PET frame.
There are also applications that require the production of several layers of graphene, also referred to as multilayer graphene. Examples of such applications are touch screen, display technology and MEMS. Multilayer graphene may be required in order to increase the electrical conductivity in transparent electrodes or the signal intensity in the graphene membrane. Multilayer graphene may be required to be suspended or deposited on a flat substrate. It may also be required to be suspended or deposited on cavities or on holes. It may also be required to be suspended or deposited on soluble substrates.
Nowadays, bilayer graphene is obtained by stacking two single layers of graphene as follows, as schematized in FIG. 1: Starting from a layer of graphene G1 deposited on a metal foil M1, a layer of PMMA PMMA1 is coated onto the graphene G1. The metal M1 underneath the graphene G1 is etched away by using a suitable etchant. The PMMA/graphene stack is then transferred onto the target substrate S1 and the PMMA layer PMMA1 is removed using a solvent. The graphene/substrate stack (G1/S1) can then receive a new PMMA/graphene stack (PMMA2/G2) obtained in a similar way (by etching the metal M2 underneath a graphene layer G2). A PMMA/graphene/graphene/target substrate stack (PMMA2/G2/G1/S1) is thus obtained. This method is called layer by layer method (LBL method). However, the PMMA residues which are left during the removal of the PMMA coating PMMA1 prior to attaching the second graphene layer G2 on the first graphene layer G1 may cause different problems, such as decreasing the contact surface between the two graphene layers, leading to poorer electrical performance (higher electrical resistivity) which is especially detrimental for all the applications. Besides, etchants used for removing the metal M1 M2 underneath the graphene G1 G2 also cause impurities to be trapped between adjacent graphene layers G1 G2.
Cheng Jin An et. al. describe in “Ultraclean transfer of CVD-grown graphene and its application to flexible organic photovoltaic cells”, Journal of Materials Chemistry A, 2014, 2, 20474-20480, a graphene transfer method, in which the PMMA/graphene stack is transferred in a reversed manner onto target substrates. According to this method, PMMA is spin coated onto highly uniform graphene prepared by CVD and the metal underneath the graphene is etched away by using FeCl3 etchant, resulting in the PMMA/graphene stack floating above the solution. Subsequent graphene layers are stacked by placing the PMMA/graphene on top of a graphene/Cu followed by etching of the Cu. After cleaning with pure water, the PMMA-coated graphene/graphene stack is transferred in a reversed manner onto the target substrate by placing the target substrate against the side of the PMMA. The resulting layers are stacked in the order graphene-graphene-PMMA-target substrate. However, it has been observed that the roughness and lack of uniformity of the PMMA layer is problematic in the case of substrates with a low roughness (Si/SiO2, certain polymers, etc.) and this may induce too much roughness into the graphene layer, leading to a worsening of its properties (electrical, thermal, etc.). In addition, using this method impurities coming from the Cu etchant and other solvents may become trapped between the two or more graphene layers leading to problems in the final applications, such as shortcuts in organic light emitting diodes (OLEDs), increased roughness for the transparent electrodes and interferences in sensor measurements.
Jing-Jing Chen et al. in “Fabrication and Electrical Properties of Stacked Graphene Monolayers”, Scientific Reports, 4:5065, DOI: 10.1038/srep05065 (27 May 2014) have developed a method for producing two-stacked graphene monolayers without any PMMA between the graphene layers. First, a PMMA thin layer is spin-coated on a monolayer graphene surface grown on copper foil. The copper foil is then dissolved by FeCl3 saturated solution for 30 min. The graphene/PMMA film is washed three times by 60° C. deionized (DI) water. Another monolayer graphene on copper foil is used to fish out the graphene/PMMA film from deionized water. Because of the face-to-face superposition of clean graphene surfaces, there is no PMMA between the graphene layers. The copper foil is then dissolved, after which the two stacked graphene monolayers/PMMA can be transferred onto an arbitrary substrate. However, Cu etchant and other solvent residues remaining between layers, coming from the transfer process, may cause different problems, such as worsening of the graphene properties, which is especially detrimental in applications in which graphene is to be used as the transparent electrode in displays, light emitting diodes/organic light emitting diodes LEDs/OLEDs (creating shortcuts, bad OLED performance) or graphene is to be deposited on soluble substrates or on cavities or holes. Thus, when removing Cu from the PMMA/graphene/graphene/Cu sample prior to its transfer to a substrate, etchants used for removing the metal underneath the graphene cause impurities to be trapped between adjacent graphene layers.
There is therefore a need to obtain multilayer graphene in which the adjacent layers of graphene are free of impurities coming from the metal, solvent, etc. Especially if graphene is to be used in semiconductor industry applications (sensors, photonics, optoelectronics, etc.), in which several layers of graphene are usually required.